Summon Japan / Nihonkoku Shoukan
*In the Great Orient, lies the continent called Rodenius.
One day, an unidentified flying object arrived at Qua-Toyne Principality’s airspace. The unidentified flying object defied the Principality’s common sense. But that unidentified flying object was just a harbinger of something even more shocking.
Far to the eastern sea, a group of islands suddenly appeared. The country of the islands called itself: Japan. According to them, their nation had been transported to another world.
Now finding themselves separated from their old world and must survive in the new one, how will Japan interact with the native countries of this world? Will they greet her as a friend or as an enemy?
This is a story of the country called Japan as it was engulfed in the chaotic storm of the world.*
The whole of Japan gets isekai-ed. If you liked Gate, you should like this one as well.
WN still going on but LN has more details and bonus materials.
I quite enjoy this series. One unusual aspect about it: there is no protagonist. It occasionally takes specific points of view, but is mostly told from a third-person perspective and never focuses on a single person for too long. I suppose you could argue that Japan is the protagonist.
One more thing - someone started doing "United States Summon"
Is it the same author or is it fanfic?
I also support this. From what I see from the manga and the story summary, this is very interesting to me.